Three types of rock specimens, three point bending specimen, anti symmetric four-point bending specimen and direct shearing specimen, were used to achieve Mode Ⅰ, Mode Ⅱand mixed mode Ⅰ2 Ⅱfracture, respectively. Microscopic characteristics of the three fracture modes of brittle rock were studied by SEM technique in order to analyze fracture behaviors and better understand fracture mechanisms of different fracture modes of brittle rock. Test results show that the microscopic characteristics of different fracture modes correspond to different fracture mechanisms. The surface of Mode Ⅰfracture has a great number of sparse and steep slip steps with few tearing ridges and shows strong brittleness. In the surface of Mode Ⅱfracture there exist many tearing ridges and densely distributed parallel slip steps and it is attributed to the action of shear stress. The co action of tensile and shear stresses results in brittle cleavage planes mixed with streamline patterns and tearing ridges in the surface of mixed mode Ⅰ Ⅱfracture. The measured Mode Ⅱfracture toughness KⅡC and mixed mode Ⅰ Ⅱfracture toughness KmC are larger than Mode Ⅰfracture toughness KⅠC. KⅡC is about 3.5 times KⅠC, and KmC is about 1.2 times KⅠC.